Geometry and evolution of the Mesoproterozoic Irumide Belt of Zambia

Abstract

AbstractThe Irumide Belt of eastern Zambia is one of a number of complex Proterozoic orogenic belts found in the African continent. The belt lies along the southeastern flank of the Tanzania-Bangweulu Craton and to the NW of the Mozambique Belt of East Africa. The dominant tectonic features of the belt are ductile shear zones, thrusts and folds related to a Mesoproterozoic, NW-SE crustal shortening event, and late-tectonic granite emplacement. Later tightening of Irumide structures and localized deformation of granites is also widespread. The timing of post-Irumide deformation is poorly constrained but is believed to be a mix of Pan-African and Late Palaeozoic reactivation and thermal reworking. This complex history is recently overprinted by significant Tertiary rifting. The Irumide deformation is evident in three tectonic zones stretching from the NW foreland of Mporokoso to the SE hinterland of Lundazi: (1) the NW-facing basement shear zones and associated cover deformation of the Luongo Fold Belt; (2) the Foreland Fold Belt of large upright folds defined by the kilometre-thick quartzite ridges of the Irumi, Shiwa Ngandu and Isoka hills; (3) an intensely deformed Internal Zone of basement and granite gneisses folded into major domes and associated tight synclines. Post-tectonic granites intrude the last of these zones and define an end to the Irumide deformation. The linking of these three zones is based on geometric, kinematic and geochronological evidence. The main Irumide deformation and metamorphism is believed to have occurred between 1100-950 Ma with a metamorphic peak between 1050 and 1000 Ma. Termination of Irumide deformation is dated by the emplacement of the post-tectonic granite and K/Ar biotite and hornblende closure dates of c. 950 Ma. Similar K/Ar dates from the Luongo shear zone indicate biotite closure and cooling during the period 1020-950 Ma. Together with parallel kinematic indicators, this supports a coeval development and tectonic link between the Irumide Belt and the Luongo Fold Belt. The intracontinental Irumide Belt deformation was driven by an accretionary collisional margin, to the SE of the present-day Irumide Belt, in southern Malawi and Mozambique. This accretionary margin is today represented by arc and island-arc magmatism and the complex terranes of the Mozambique Belt of East Central Africa.